1. Field of the Invention
This invention relates to coating of semiconductor devices and more particularly to a method and apparatus for coating light emitting diodes (LEDs) with a matrix material containing one or more light conversion materials.
2. Description of the Related Art
LEDs are solid-state devices that convert electric energy to light and they generally comprise an active layer of semiconductor material sandwiched between two oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light that is emitted omnidirectionally from the active layer and from all surfaces of the LED. Recent advances in LEDs (such as Group III nitride based LEDs) have resulted in highly efficient light sources that surpass the efficiency of filament-based light sources, providing light with equal or greater brightness in relation to input power.
One disadvantage of conventional LEDs used for lighting applications is that they cannot generate white light from their active layers. One way to produce white light from conventional LEDs is to combine different wavelengths of light from different LEDs. For example, white light can be produced by combining the light from red, green and blue emitting LEDs, or combining the light from blue and yellow LEDs.
One disadvantage of this approach is that it requires the use of multiple LEDs to produce a single color of light, increasing the overall cost and complexity. The different colors of light are also generated from different types of LEDs fabricated from different material systems. Combining different LED types to form a white lamp can require costly fabrication techniques and can require complex control circuitry since each device may have different electrical requirements and may behave differently under varied operating conditions (e.g. with temperature, current or time).
More recently, the light from a single blue emitting LED has been converted to white light by coating the LED with a yellow phosphor, polymer or dye, with a typical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG). [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; See also U.S. Pat. No. 5,959,316 to Hayden, “Multiple Encapsulation of Phosphor-LED Devices”]. The surrounding phosphor material “downconverts” the wavelength of some of the LED's blue light, changing its color to yellow. For example, if a nitride-based blue emitting LED is surrounded by a yellow phosphor, some of the blue light passes through the phosphor without being changed while a substantial portion of the light is downconverted to yellow. The LED emits both blue and yellow light, which combine to provide a white light.
One conventional method for coating an LED with a phosphor layer utilizes a syringe or nozzle for injecting a phosphor containing epoxy over the LED. One disadvantage of this method is that it is often difficult to control the phosphor layer's geometry and thickness. As a result, light emitting from the LED at different angles can pass through different amounts of conversion material, which can result in an LED with non-uniform color temperature as a function of viewing angle. Another disadvantage of the syringe method is that because the geometry and thickness is hard to control, it is difficult to consistently reproduce LEDs with the same or similar emission characteristics.
Another conventional method for coating an LED is by stencil printing, which is described in European Patent Application EP 1198016 A2 to Lowery. Multiple light emitting semiconductor devices are arranged on a substrate with a desired distance between adjacent LEDs. The stencil is provided having openings that align with the LEDs, with the holes being slightly larger than the LEDs and the stencil being thicker than the LEDs. A stencil is positioned on the substrate with each of the LEDs located within a respective opening in the stencil. A composition is then deposited in the stencil openings, covering the LEDs, with a typical composition being a phosphor in a silicone polymer that can be cured by heat or light. After the holes are filled, the stencil is removed from the substrate and the stenciling composition is cured to a solid state.
One disadvantage of this method is that, like the syringe method above, it can be difficult to control the geometry and layer thickness of the phosphor containing polymer. The stenciling composition may not fully fill the stencil opening such that the resulting layer is not uniform. The phosphor containing composition can also stick to the stencil opening which reduces the amount of composition remaining on the LED. These problems can result in LEDs having non-uniform color temperature and LEDs that are difficult to consistently reproduce with the same or similar emission characteristics.
Another conventional method for coating LEDs with a phosphor utilizes electrophoretic deposition. The conversion material particles are suspended in an electrolyte based solution. A plurality of LEDs are arranged on a conductive substrate that is then almost completely immersed in the electrolyte solution. One electrode from a power source is coupled to the conductive substrate at a location that is not immersed in the solution, and the other electrode is arranged in the electrolyte solution. The bias from the power source is applied across the electrodes, which causes current to pass through the solution to the substrate and its LEDs. This creates an electric field that causes the conversion material to be drawn to the LEDs, covering the LEDs with the conversion material.
One of the disadvantages of this method is that after the LEDs are covered by the conversion material, the substrate is removed from the electrolyte solution so that LEDs and their conversion material can be covered by a protective epoxy. This adds an additional step to the process and the conversion material (phosphor particles) can be disturbed prior to the application of the epoxy. Another disadvantage of this process is that the electric field in the electrolyte solution can vary such that different concentrations of conversion material can be deposited across the LEDs. The conversion particles can also settle in the solution which can also result in different conversion material concentrations across the LEDs. The electrolyte solution can be stirred to prevent settling, but this presents the danger of disturbing the particles already on the LEDs.